Advances in the field of semiconductor manufacturing have decreased the achievable minimum feature size. This decrease in feature size has the undesirable side effect of increasing the capacitive coupling between adjacent devices. As the amount of interconnecting metallurgy increases, the capacitive coupling problem impedes performance. Efforts to minimize the effects of capacitive coupling include isolating wiring into levels with insulators or air gaps between the levels.
Silicon dioxide is a commonly used insulator in the fabrication of integrated circuits. As the density of devices, such as resistors, capacitors and transistors, in an integrated circuit is increased, several problems related to the use of silicon dioxide insulators arise. First, as metal signal carrying lines are packed more tightly, the capacitive coupling between the lines is increased. This increase in capacitive coupling is a significant impediment to achieving high speed information transfer between and among the integrated circuit devices. Silicon dioxide contributes to this increase in capacitive coupling through its dielectric constant, which has a relatively high value of four. Second, as the cross-sectional area of the signal carrying lines is decreased for the purpose of increasing the packing density of the devices that comprise the integrated circuit, the signal carrying lines become more susceptible to fracturing induced by a mismatch between the coefficients of thermal expansion of the silicon dioxide and the signal carrying lines.
One solution to the problem of increased capacitive coupling between signal carrying lines is to use an insulating material that has a lower dielectric constant than silicon dioxide. Polyimide has a dielectric constant of between about 2.8 and 3.5, which is lower than the dielectric constant of silicon dioxide. Using polyimide lowers the capacitive coupling between the signal carrying lines. Unfortunately, there are limits to the extendibility of this solution, since there are a limited number of insulators that have a lower dielectric constant than silicon dioxide and are compatible with integrated circuit manufacturing processes.
One solution to the thermal expansion problem is to use a foamed polymer for the insulating layer. The mismatch between the coefficient of thermal expansion of a metal signal carrying line and the coefficient of thermal expansion of a foamed polymer insulator is less than the mismatch between the coefficient of thermal expansion of a metal signal carrying line and the coefficient of thermal expansion of silicon dioxide. In addition, a foamed polymer will have an effective yield strength which is considerably lower than that of copper. Although these foamed polymers have low dielectric constants, they also have mechanical properties that are not suitable for certain steps of the semiconductor manufacturing process, such as chemical-mechanical polishing (CMP). CMP requires that the insulating layer have sufficient mechanical strength to withstand the polishing forces.